The present invention relates generally to light emitting devices that contain organic electron transport materials and more particularly to novel organic compounds for use as electron transport materials.
Early light emitting or electroluminescent devices utilized only a single layer of organic luminescent material sandwiched between two injecting electrodes. The first electrode injected holes and the second electrode injected electrons into the organic material. As described in U.S. Pat. No. 3,530,325 to Mehl et al., one such device utilized a single crystal anthracene as the organic material. The most significant disadvantage to the use of anthracene, however, is the unacceptably high drive voltage required to produce a significant or commercially viable amount of light. In fact, the required drive voltage was determined to be 100 volts or higher. Necessarily, the power-consumption efficiency of these early devices was quite low and typically less than 0.1% W/W.
The primary factor contributing to the high drive voltage was the inability to reduce the thickness of the anthracene layer below 50 xcexcm. Attempts to reduce the thickness of these and related materials in order to reduce the drive voltage, proved overall to be unsuccessful. Although some success was achieved in reducing the drive voltage below 30 volts, the reduction of the anthracene layer to 1 xcexcm or less resulted in the formation of pinholes which acted as shorts between the electrodes. These shorts virtually eliminated the amount of luminescence produced. Even the addition of non-conductive polymeric binders as a remedy for the pinholes proved generally unsuccessful due to the interference of the binders with the injection of holes and electrons. As a result, and despite lowering the required drive voltage, these devices offered only very low quantum efficiencies of about 0.05%.
More recently, double-layered light emitting devices manufactured using organic thin films have been developed. These devices typically include a cathode of a low-work-function metal or alloy for efficient electron injection, an anode of indium-tin-oxide, an aromatic diamine for the hole injecting or transporting layer and a luminescent film of aluminum tris(8-oxy-1-quinoline) (Alq3) from a class of fluorescent metal chelate complexes for the electron injecting or transporting layer. This particular double-layered device has proven to be successful in lowering the required drive voltage (approximately 6-14 volts) while maintaining sufficient quantum efficiencies of about 1.0%. Despite these advances, concern regarding the commercialization of these devices still remains and revolves primarily around their overall stability during continuous operation.
In particular, it is known that these particular double-layered light emitting devices incur a relatively fast degradation of the electroluminescence emission. More specifically, tests have shown an initial degradation of around 30% over the first ten hour period to about 50% over a one hundred hour period. Thus, a need is identified for improved organic compounds for use in these multilayered devices having a low drive voltage, high quantum efficiency and which overcome the present problem of relatively rapid degradation of the electroluminescence emission.
An important aspect of the present invention is to provide electron transport materials for use in light emitting devices that contain the electron transport material having a low drive voltage, improved thermal stability while in most cases and in most applications maintaining comparable efficiencies. In particular, the improved thermal stability, comparable efficiency and high yields are due to the utilization of novel electron transport materials for transporting electrons in the light emitting device.
Thus, in accordance with the preferred embodiment of our present invention, the light emitting device includes an electron transporting material formed as a layer in contact with a hole transporting material similarly formed as a layer. These layers are further confined between two electrodes. A power source is connected in a conventional manner to each of the electrodes. The first electrode, or cathode, is electrically connected to the electron transporting layer and the second electrode, or anode, is likewise connected to the hole transporting layer. When the cathode is negatively biased in relation to the anode, electrons are injected into the electron transporting layer. The concurrent positive bias on the anode in relation to the cathode causes holes to be injected from the cathode into the hole transporting layer. Electroluminescence is produced and confined generally near the interface between the electron and hole transporting layers as a result of the recombination of the electron and hole pairs.
In accordance with an important aspect of the present invention, the electron transporting layer is an aluminum chelate prepared from various polycyclic aromatic compounds selected from a group including hydroxyphenalenones, hydroxybenzanthrones, phenalenes, hydroxybenzanthracenones and mixtures thereof including particularly phenalenone and/or a benzanthrone based precursors including, for example, the following compounds: 
In the most preferred embodiments, the electron transporting layer is formed from tris-(9-oxidophenalenone) aluminum [Al(9-opo)3] and/or a benzannellated derivative thereof, tris (6-oxidobenzanthrone) aluminum [Al(6-obao)3], compounds 1 and 4 respectively. These compounds are electron acceptors having improved thermal stability while in most cases and in most applications maintaining comparable efficiencies.